Description:
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February is usually 28 days and leap 29 days a year. The problem is that when calling the script, we should get an equal two months instead of one month and 28 days. The situation also has when we call for 2020, then we will receive 1 day and 29 days. For the remaining months in which February does not exist, DateInterval returns the correct result as the number of months assuming the first day of the month.

The probable errors occurs in all PHP version in 7.1 + what show a 3v4l.org
https://3v4l.org/KeTaO

Test script:
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<?php

$lastDate = new DateTime('2019-03-01');
$firstDate = new DateTime('2019-01-01');

$dateInterval = $lastDate->diff($firstDate);

var_dump($dateInterval);

Expected result:
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object(DateInterval)#3 (16) { ["y"]=> int(0) ["m"]=> int(2) ["d"]=> int(0) ["h"]=> int(0) ["i"]=> int(0) ["s"]=> int(0) ["f"]=> float(0) ["weekday"]=> int(0) ["weekday_behavior"]=> int(0) ["first_last_day_of"]=> int(0) ["invert"]=> int(1) ["days"]=> int(59) ["special_type"]=> int(0) ["special_amount"]=> int(0) ["have_weekday_relative"]=> int(0) ["have_special_relative"]=> int(0) }

Actual result:
--------------
object(DateInterval)#3 (16) { ["y"]=> int(0) ["m"]=> int(1) ["d"]=> int(28) ["h"]=> int(0) ["i"]=> int(0) ["s"]=> int(0) ["f"]=> float(0) ["weekday"]=> int(0) ["weekday_behavior"]=> int(0) ["first_last_day_of"]=> int(0) ["invert"]=> int(1) ["days"]=> int(59) ["special_type"]=> int(0) ["special_amount"]=> int(0) ["have_weekday_relative"]=> int(0) ["have_special_relative"]=> int(0) }